The present invention relates to a method for producing a compound semiconductor single crystal, for example, to a useful technique applied to a vertical gradient freeze method (VGF) or vertical Bridgman (VB) method for growing a single crystal in a vertical direction by cooling raw material melt of a compound semiconductor, such as InP.
Previously, for producing industrially a compound semiconductor single crystal ingot of group III-V, such as GaAs, InP or the like, for example, a liquid encapsulated Czochralski (LEC) method or a horizontal Bridgman (HB) method has been generally applied.
While the LEC method has advantages that crystal orientation is controllable by seeding, a wafer having a large diameter and a circular shape in sectional form can be obtained, and a high purity crystal can be easily obtained by using a liquid encapsulating material (B2O3), the method has disadvantage that a large temperature gradient in a direction of crystal growth makes thermal stress to the crystal large, so that etch pit density (EPD) in the crystal becomes high. Accordingly, in a case of making an optical device by using the crystal, a problem that a property deteriorates due to a crystal defect is occurred.
On the other hand, while the HB method has an advantage that a crystal having low etch pit density can be obtained due to a small temperature gradient in the direction of crystal growth, the method has disadvantages that making the diameter of wafer large is difficult and further, the only wafer having a shape depending a shape of a crucible (for example, a vault-like shape) can be obtained, because the raw material melt is solidified in the crucible (boat).
As a method for producing a single crystal, which complements the respective disadvantages of the HB method and the LEC method and has the respective advantages thereof, the vertical gradient freeze (VGF) method and the vertical Bridgman (VB) method have been developed.
With the VGF method and the VB method, a temperature of a crystal growth furnace is lowered to gradually cool compound semiconductor raw material melt contained in a refractory crucible, thereby the compound semiconductor single crystal is grown. Therefore, these methods have advantages that the thermal stress is small and the etch pit density is small, because the temperature gradient in the melt during the crystal growth is tens to several xc2x0 C./cm, which is one digit smaller than the LEC method.
In a case of the VGF method, a prior crystal growth apparatus will be explained based on FIGS. 8 and 9. FIG. 8 is a sectional view of a crystal growth apparatus used for the prior VGF method, and FIG. 9 is a schematically sectional view of a lower portion of another crystal growth apparatus.
The crystal growth apparatus shown in FIG. 8 has a hot zone Z surrounded by a cylindrical refractory wall 113 and a top cover 114 in a high pressure container 110. In the hot zone Z, heaters 111, for example, graphite heaters are disposed along inner peripheries thereof. An inner container 105 is set on an inside of the heaters 111 with being supported by a lower shaft 112. A refractory crucible 101, such as a pBN crucible is disposed in an inside of the inner container 105, with being supported by a crucible supporting base 106. A seed crystal set portion 102 having a bottom and a cylindrical shape is formed at a bottom of the crucible 101. The inner container 105 containing the crucible supporting base 106 in the bottom portion and the lower shaft 112 supporting the inner container 105 constitute a crucible supporting means. A thermocouple 107 for measuring a temperature of a seed crystal is provided in the crucible supporting base 106, in the vicinity of the seed crystal set portion. A reference numeral 108 denotes a cover made of quartz and sealing the inner container 105 with fitting.
A seed crystal S is contained in the seed crystal set portion 102. A compound semiconductor raw material 103 and an encapsulating material 104 are contained in the crucible 101.
In the crystal growth apparatus, the compound semiconductor single crystal is grown by heating the hot zone Z, melting the compound semiconductor raw material 103 in the crucible 101 to be raw material melt, and by gradually cooling the raw material melt of the compound semiconductor so that a single crystal will grow upward from a lower portion of the crucible 101.
On the other hand, with the crystal growth apparatus shown in FIG. 9, a cylindrical refractory wall 153 is set in a high pressure container 151 and heaters 154 are buried in the cylindrical refractory wall 153. In an inside of the cylindrical refractory wall 153, a furnace tube 152 is provided, and in an interior thereof, a crucible 156 is fixed and supported by a susceptor 155 as a crucible supporting means. The susceptor 155 has a crucible supporting portion 155a and a lower shaft portion 155b. In this example, the interior of the furnace tube 152 constitutes a hot zone Z. In the crystal growth apparatus, the susceptor 155 is heated by the heaters 154, and the crucible 156 is heated by the heated susceptor 155 to melt the compound semiconductor raw material in the crucible to be the raw material melt. Then, the raw material melt of the compound semiconductor is gradually cooled so that the single crystal will grow upward from a lower portion of the crucible 156, thereby the compound semiconductor single crystal is grown.
According to the VGF method and the VB method, the wafer having the circular shape can be obtained by using the crucible having the bottom and the cylindrical shape. Further, the compound semiconductor raw material melt contained in the crucible is gradually cooled to grow the compound semiconductor single crystal, with lowering the temperature of the crystal growth furnace, thereby the temperature gradient in the direction of the crystal growth becomes small, so that making the etch pit density of the grown crystal low can be easily achieved.
However, the VGF method and the VB method are easy influenced by a slight change of the temperature in the reaction furnace, by an unevenness on an inner wall of the crucible or by contaminants attached to the inner wall. Thus, crystal defect portion, such as a twin or polycrystal is generated in a crystal diameter increasing portion which is from an initial point of the crystal growth in the crucible to a body portion thereof. These are main causes to lower a yield of the single crystal production.
Among these defects, the polycrystal generation due to the influence of the unevenness of the crucible or the contaminants attached thereto comes to be prevented by using the liquid encapsulating material (B2O3) during the crystal growth. On the other hand, the slight change of the temperature in the furnace comes to be solved by recent progresses of the temperature control technology. However, a method for effectively preventing the twin generation in the crystal diameter increasing portion from the initial point of the crystal growth to the body portion has not been developed.
In a case of growing a compound semiconductor single crystal of zinc-blende structure, such as GaAs or InP by using a seed crystal, it has shown that there is a closely relation between a tilt angle of the diameter increasing portion from the seed crystal to the body portion and probability of generation of the twin.
That is, in a case of growing a (100)-oriented crystal, a (111) facet plane appears in the diameter increasing portion and the twin generates from the facet plane. This phenomenon is verified by the experiments carried out by the present inventors. According to the experiments by the presest inventors, all twins generate along the facet plane of the crystal in which the twins generate.
An angle between the (111) facet plane and (100) orientation is 54.7xc2x0. Generally, in order to prevent appearance of the (111) facet plane, the tilt angle of the diameter increasing portion of the crucible is made (90xc2x0-54.7xc2x0), that is, which is smaller than 35.30xc2x0.
However, if the tilt angle of the diameter increasing portion is made small, a diameter increasing portion of the grown crystal also becomes long. Therefore, there is a problem that the productivity reduces because a yield of wafer lowers.
There is a report that carrying out of the crystal growth with the tilt angle of 40xc2x0 to 50xc2x0, of the diameter increasing portion of the crucible is efficient (Semiconductor investigation, volume 35, page 4, or the like). However, the present inventors carried out supplementary experiments, thereby it was found that the twin generation was difficult to be sufficiently suppressed when the tilt angle of the diameter increasing portion was within 30xc2x0 to 50xc2x0.
The present applicant (Japan Energy Corp.) proposed a method for producing a single crystal, wherein a crucible of which a bottom tilts to make a determined angle of not less than 80xc2x0 and less than 90xc2x0 to a vertical direction so as to gradually lower toward the center of the bottom is used to suppress the twin generation in a case of producing a compound semiconductor single crystal with the VGF method or the VB method (Japanese Patent Application No. Hei 9-119069).
With the producing method, the bottom of the crucible was about flat, thereby it became a state that there was substantially little diameter increasing portion from the seed crystal to the body portion. Then, time of the crystal growth on the diameter increasing portion where the twin tended to generate became extremely short, so that the drastic decrease of the provability of the twin generation was expected.
However, with respect to InP, the twin tends to generate physically compared with GaAs, thereby it was difficult to sufficiently suppress the twin generation in the diameter increasing portion by the producing method when the crystal growth was carried out with the VGF method or the VB method. Therefore, with respect to the production of the InP single crystal, a method has proposed, in which a seed crystal having approximately the same diameter as that of the growing crystal with the VGF method or the VB method is used for making physically no diameter increasing portion from the seed crystal to the body portion so as not to generate the twin.
However, with the above-described method, the seed crystal having approximately the same diameter as that of the growing crystal is required, thereby there is a problem that a wasteful seed crystal portion is large, which causes to increase a production cost, so that it is not practical for the industrial production.
Further, when the growth of the compound semiconductor single crystal is carried out in a high pressure container with the VGF method or the VB method, with introducing high pressure inert gas thereinto, there is a problem that the crystal defect portion, such as a twin or polycrystal generates in the seed portion or in a portion extending from the seed portion to the body portion, which is one of factors lowering the yield of the single crystal production.
With respect to this problem, it is caused by a temperature fluctuation during the growth in the apparatus, which is found by studies by the present inventors.
That is, in the FIG. 8, the arrows G show a gas flow in the high pressure container, simulated by a computer. As shown in the figure, it was ascertained that the gas flow of different directions collided at approximately the same speed, thereby the gas flow tended to disturb, which was a cause to occur the temperature fluctuation.
The present invention was developed in view of the above-described problems. An object of the present invention is to provide a method for producing a compound semiconductor single crystal, which effectively prevents generation of a twin or polycrystal in a seed portion or a diameter increasing portion extending from the seed portion and can produce a compound semiconductor single crystal with a higher yield, particularly the one, such as InP in which a twin tends to generate.
Hereinafter, the content of consideration and the processes of study until the inventors achieved the present invention will be explained briefly.
At first, the inventors proposed that carrying out of the crystal growth with using a crucible having an about flat-shaped bottom to grow the crystal without forming to the utmost the diameter increasing portion in which the provability of the twin generation is high. With respect to the proposal, the present inventors investigated an optimum condition for the crystal growth in the crystal diameter increasing portion.
Generally, an interface between solid and melt during the crystal growth is prefer that the solid phase is convex-shaped to easily become single crystal. For that purpose, the speed of the crystal growth is desired to be slow to the utmost. However, on the results of experiments carried out by the present inventors, even if the speed of the crystal growth was made slow, the state of the interface between solid and melt in the crystal diameter increasing portion was not so improved, so that it was difficult to effectively suppress the twin generation.
As other manners for improving the interface between solid and melt, the one is proposed that a holding material for the crucible is properly changed, the seed crystal portion is forced to be cooled or the like. However, according to the experiments by the present inventors, none of the manners could not certainly suppress the twin generation.
From the results obtained by the experiments for growing the GaAs single crystal with the VGF method, the present inventors studied and investigated a relation between positions where the twin generated in the crucible and (111) facet patterns shown in the crystal diameter increasing portion to find that the (111) facet patterns related to the twin generation.
That is, among the GaAs single crystals grown with the VGF method, while no twin generated in a case that the facet patterns were nearly a square, the twin generation was often observed in a case that the facet patterns were unsymmetry, such as that only a facet in one direction became long. The generation of these unsymmetrical facet patterns shows that the solidification of the melt in the diameter increasing portion does not proceed in four directions at a uniform rate, but proceeds rapidly in one direction. It is adequately possible that such condition of the facet growth influences the twin generation.
The present inventors, based on the consideration described above, considered that a condition of the crystal growth, by which the facet growth would proceed with good symmetry was required for certainly suppress the twin generation. As a result of many studies, the present inventors have been achieved to obtain a conviction that the speed of the crystal growth, which is faster than a determined value is effective to grow the facet with good symmetry.
Previously, in the VGF method, because the temperature gradient in the vicinity of the interface between solid and melt is small, the rate of the crystal growth of approximately several mm/hr has been desirable although it slightly differs according to the thermal conductivity of the growing crystal.
The present inventors, with respect to InP having a physical property that the twin tends to generate therein, tested the crystal growth with varying the growth rates to 5 mm/hr, 10 mm/hr, 20 mm/hr, or 50 mm/hr at the diameter increasing portion, with being free from the above-described rate of the crystal growth. Although the twin generated in the portion from the seed crystal to the diameter increasing portion at the growth rates of 5 mm/hr, 10 mm/hr, and 20 mm/hr at the diameter increasing portion, it was gradually decreased and it was firstly achieved to obtain the InP single crystal having no twin when the growth rate was 50 mm/hr at the diameter increasing portion.
However, in the case that the growth rate was 20 mm/hr at the diameter increasing portion, the twin generated in a small portion of edge, so that the InP single crystal of high quality could be obtained in most crystal body portion.
Based on the results, the conditions of the growth rate for industrially obtaining the single crystal of InP or the like with a high yield are studied and it comes to conclusion that a desirable rate of the crystal growth at the diameter increasing portion (shoulder portion) is at least not less than 20 mm/hr, preferably not less than 30 mm/hr, more preferably not less than 40 mm/hr, most preferably not less than 50 mm/hr. The present invention is achieved based on the above-described knowledge.
As described above, in one aspect of the present invention, a method for producing a compound semiconductor single crystal comprises the steps of: using a crucible having a bottom, a cylindrical shape, a diameter increasing portion having a reversed conical shape in a lower end side of the crucible, and a set portion for a seed crystal in a center of the bottom of the diameter increasing portion; setting a seed crystal in the seed crystal set portion of the crucible; putting a raw material of the compound semiconductor and an encapsulating material into the crucible; enclosing the crucible in an inner container; thereafter setting the inner container in a vertical type furnace; heating the raw material and the encapsulating material by a heating means to melt; and solidifying the obtained raw material melt from the seed crystal toward an upper side with annealing the raw material melt from a lower side to grow a single crystal of the compound semiconductor; wherein a rate of crystal growth at the diameter increasing portion of the crucible is made not less than 20 mm/hr during the crystal is grown.
It may be preferable that the diameter increasing portion having the reversed conical shape, of the crucible has a determined tilt angle of not less than 40xc2x0 and less than 90xc2x0 to a normal line of a center of the bottom, more preferably, not less than 60xc2x0 and less than 90xc2x0, and especially, not less than 80xc2x0 and less than 90xc2x0.
A temperature gradient in a direction of the crystal growth at least at the diameter increasing portion of the crucible may be controlled to have 1 to 10xc2x0 C./cm during the crystal growth.
The vertical type furnace can comprise a high pressure container and a hot zone surrounded by a cylindrical refractory wall and an upper cover, disposed in the high pressure container.
According to the method for producing a compound semiconductor single crystal of the present invention, twin generation in the diameter increasing portion of the crystal can be effectively prevented. Thereby, a compound semiconductor single crystal can be obtained with a high yield, particularly, the compound semiconductor single crystal, like InP in which a twin tends to generate, can be produced efficiently and with a low cost.
In order to solve the problem that the gas of different directions and having approximately the same speed collides, thereby the flow tends to disturb and the temperature fluctuation (a variation between a maximum temperature and a minimum temperature in changes of temperature which rises and falls in the setting temperature in a determined period) becomes large, so that the crystal defect portion, such as a twin, polycrystal or the like tends to generate in the diameter increasing portion of the crystal in the prior apparatus, the present inventors proposed that a laminar flow heading upward from a lower side around the crucible is forced to generate to reduce the influence of a turbulent flow generated by the collision of the gas of different directions. As a result of studies, it came to conclusion that it is optimum that a hole (air hole) is formed in an upper cover for a hot zone to generate convection through the hole in whole inside of the apparatus.
Further, it was found that previously, the bottom of the crucible supporting portion of the crucible supporting means had structure which was flat or nearly flat, thereby, if the crystal growth was carried out in the high pressure container, the gas flow crashing to the bottom was disturbed to become the turbulent flow, which was a cause of a fluctuation of the temperature of a system. It was also found that for preventing this, it was effective that the bottom was formed into a cone-like (cone) shape so that the gas flow around the lower end peripheral surface of the supporting portion would flow along a streamline shape.
In another aspect of the present invention, a method for producing a compound semiconductor single crystal, which has been accomplished based on the above-described knowledge, in which raw material melt of the compound semiconductor contained in a refractory crucible is annealed from a lower side in a hot zone surrounded by a cylindrical refractory wall and an upper cover in a high pressure container to grow the compound semiconductor single crystal, the method comprises the steps of: inducing a gas flow heading upward from a lower side around peripheries of the crucible by a gas flow inducing means; and growing a single crystal of the compound semiconductor, with reducing a temperature fluctuation.
More concretely, the gas flow inducing means can comprise a single or a plurality of air holes (holes) formed in the upper cover for the hot zone, thereby the gas flow heading upward from the lower side around the peripheries of the crucible through the air hole can be induced. Then, the compound semiconductor single crystal can be grown with reducing the temperature fluctuation.
Further, the gas flow inducing means can comprise a single or a plurality of air holes (holes) formed in the upper cover for the hot zone and an opening portion formed in a lower end side of the hot zone, and induce convection which heads upward from the lower side around the peripheries of the crucible, passes through the air holes of the upper cover, heads downward from an upper side around peripheral portions of the cylindrical refractory wall, passes through the opening portion and comes back to the peripheries of the crucible through peripheries of the supporting means. Thereby, the compound semiconductor single crystal can be grown with reducing the temperature fluctuation.
A lower end peripheral surface of the inner container can be formed into a cone-like shape, or the supporting means can comprise a susceptor having a crucible containing portion and a lower shaft, wherein a lower end peripheral surface of the crucible containing portion of the susceptor can be formed into a cone-like shape. Thereby, the gas flow can be changed into a laminar flow, so that the temperature fluctuation can be further reduced. A conical angle of the lower end peripheral surface can be within 20 to 80 degrees.
According to the invention, the temperature fluctuation can be reduced to not more than xc2x10.05xc2x0 C., which is approximately {fraction (1/10)} of that in the prior case, so that generation of crystal defect portion, such as a twin or polycrystal or the like can be suppressed.